
class Solution:

    def mergeTwoLists(self, l1, l2):

        # 递归结束条件
        if not l1:
            return l2
        
        if not l2:
            return l1
        # 递归
        if l1.val <= l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)

            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)

            return l2


